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<TITLE>A Phone Number Formatting Facet Class</TITLE>
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<H2>26.3 A Phone Number Formatting Facet Class</H2>
<P>Now that we have locales and facets in C++, we can encapsulate the locale-dependent parsing and formatting of telephone numbers into a new facet class. Let's focus on formatting in this example. We call the new facet class <SAMP>phone_put</SAMP>, analogous to <SAMP>time_put</SAMP>, <SAMP>money_put</SAMP>, etc.</P>
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<P>The <SAMP>phone_put</SAMP> facet class serves solely as a base class for facet classes that actually implement the locale-dependent formatting. The relationship of class <SAMP>phone_put</SAMP> to the other facet classes is illustrated in <A HREF="26-3.html#Figure&nbsp;13">Figure&nbsp;13</A>:</P>
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<H4><A NAME="Figure&nbsp;13">Figure&nbsp;13: The relationship of the phone_put facet to the implementing facets</A></H4>

<P><IMG SRC="images/stdlibug-Locales11.gif" WIDTH=555 HEIGHT=226></P>
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<P>Here is a first tentative declaration of the new facet class <SAMP>phone_put</SAMP>:</P>

<UL><PRE>
class phone_put: public std::locale::facet {                 // 1
public:
  static std::locale::id id;                                 // 2
  typedef std::string string_type;

  phone_put (std::size_t refs = 0)
    : std::locale::facet (refs) { }                          // 3

  string_type put (const string_type &amp;cntryName,
                   const string_type &amp;areaCode,
                   const string_type &amp;extension) const;      // 4
};

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<TABLE CELLPADDING="3">

<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>Derive from the base class <SAMP>std::locale::facet</SAMP>, so that a locale object is able to maintain instances of our new phone facet class.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>New base facet classes need to define a static data member <SAMP>id</SAMP> of type <SAMP>std::locale::id</SAMP>.
<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>Define a constructor that takes the reference count that is handed over to the base class.
<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>Define a function <SAMP>put()</SAMP> that does the actual formatting.
</TABLE>

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